The present invention relates to the field of optical and magneto-optical information storage systems and, more particularly, to portions of an optical assembly for applying a focused light beam to information storage media for recording or retrieving information optically.
Information storage systems, particularly those used with computer systems, typically store data magnetically or optically onto several types of storage media, rotating magnetic or optic disks for example. Such storage media may include those information memory media for document files, computer output memories and the like which are used for recording and retrieval only, or media which permit recording, retrieval and erasure of information. The data stored on such media, whether magnetic or optical, is contained within a series of tracks. Once formed on a disk, such tracks are spiral or concentric about the disk center and may number into the thousands of tracks per disk side. The total number of tracks and hence the storage capacity of the disk depends on the diameter of the disk utilized and the method of recordation, either magnetically or optically, of the data.
In both magnetic recording and magneto-optical recording, information is stored on a storage disk by orienting the magnetic field of the media at given points along given tracks. In order to record, access and read data on a disk, a read-write head or transducer for magnetic recording or an optical assembly comprising at least an objective lens in the case of magneto-optical recording is moved along a generally radial path across the surface of the storage disk as the storage disk is being rotated. The generally radial movement of the transducer or optical assembly will either follow a straight line path or an arcuate path, depending upon whether a linear or rotary actuator is utilized to position the head.
In magneto-optical storage, data are recorded and erased on a thin film of magnetic material which is deposited on a substrate of suitable material. In magneto-optical recording information is encoded and stored in a sequence of magnetic domains oriented normal to the storage media surface in either of two possible orientations, north pole up or north pole down for example. An erased track has all of its magnetized regions or domains oriented in one direction. Typically, on magneto-optic media, the magnetic force required to reverse or flip a magnetic domain from, for example, north pole down to north pole up, i.e., the coercive force required, varies greatly with the temperature of the media. At room temperature the coercive force necessary to reverse the magnetic domains is very high and therefore requires an extraordinarily large magnet. At approximately 150 degrees C. the coercive force required to reverse a magnetic domain decreases substantially and a domain may be flipped or reversed using ordinary magnets including electromagnets.
During a recording operation, a focused laser beam is used in a magneto-optic system to heat selected, localized spots on the recording media in a magnetic field. In this manner a point on the recording media can be heated, thereby lowering the coercive force required to write information and the magnet, depending on the direction of magnetic flux generated by such magnet, will cause the orientation of the magnetic domain to be reversed locally thereby recording the desired information. When the laser beam is turned off, the previously heated spot on the media cools, "freezing" the orientated domain in the desired orientation. To erase information so recorded the process need only be reversed; i.e., the point on the media will be heated by the laser beam and the direction of magnetic flux generated by the magnet will be such to reorient or reverse the magnetic domain.
In optical and magneto-optical recording and reading apparatus, an optical assembly is employed which applies the light beam to a data storage media. Such optical assemblies must meet a number of precisely defined needs which identify them as high precision devices when compared with common optical devices such as cameras, microscopes and the like. In this optical assembly a laser beam typically generated by a semiconductor laser or other suitable light source is focused on the storage medium by an objective lens. The light beam may be either reflected by the media or transmitted through the media. The laser beam when reflected from the media passes again through the objective lens and is then detected by a photodetector or other suitable detector. A detected signal is then processed to extract the information contained therein. A second function of the optical assembly is to derive tracking and focusing signals. Typically, prior art optical assemblies of this type are provided with a focusing servo mechanism to detect and maintain a focusing state for the objective lens and a tracking servo mechanism for detecting a tracking guide, continually directing the objective lens towards a desired track. Typical prior art optical assemblies can include large numbers of components and require a complex assembly and adjustment process and can be bulky and massive.
U.S. Pat. No. 4,783,589 issued on Nov. 8, 1988 and entitled "Focus and Tracking Detection Apparatus for Optical Head Employing Light Guide Means Having Different Radii of Curvature" describes an optical assembly for use with an optical memory. The described optical assembly includes optical means for directing a light beam to the optical storage media and means for separating the resultant light beam reflected from the optical storage media into first and second light beams. The optical assembly further includes first and second light beam emerging surfaces having different radii of curvature from which the first and second light beams emerge. While the described optical assembly is more compact and less bulky than other prior art optical assemblies, it is complex, requiring the first and second light beam emitting surfaces to have different radii of curvature and an additional lens to converge the resultant first and second light beams.